Students must use oral and written explanations to demonstrate their grasp of academic content, but little is known about the progression of elementary students' explanatory language, particularly oral language. The current study of 512 explanations produced by 128 third- through sixth-grade students examines whether children's oral explanations differ from their written explanations in terms of vocabulary, sentence structure, and features of discourse. Students were asked to explain academic and nonacademic tasks in both modes (oral and written). Multilevel ordered logistic regression controlled for type of task, grade level, English learner status, and gender and examined interaction effects with mode. Analyses found that students' oral explanations demonstrated more sophisticated sentence structure and greater coherence/cohesion than their written explanations. Interaction effects revealed several contextual factors that play a key role in understanding differences between the two modes. We discuss implications for instruction, assessment, and future research.